Molecular logic proposed

By
Eric Smalley,
Technology Research NewsResearchers
from the French National Center for Scientific Research (CNRS) and University
College London in England have devised a scheme for designing logic circuits
within individual molecules.

The scheme could eventually be used to produce small, fast computers
and to store large amounts of data in very small spaces. The method could
also be modified to make sensors for detecting individual molecules.

The researchers' plan calls for connecting a pair of benzene molecules
to two gold electrodes. The molecules contain nitrogen-oxygen side groups
whose rotational positions can represent the 1s and 0s of computer information.
The researchers' simulations show that the set-up would allow for simple
two-input logic gates like AND and XOR, said Robert Stadler, an assistant
professor of physics and astronomy at University College London.

An AND gate contains two inputs and one output. If the inputs are
both 1s the gate returns an output of 1. If the inputs are different or
both 0s, the output is 0. An XOR gate returns a 1 if either but not both
of the inputs are 1.

The molecular circuits also have the potential to perform more complex
functions, said Stadler. "We envision [a] move towards complexities where
a far larger number of inputs can be processed through a single molecule,"
he said.

The researchers hit upon the idea while they were trying to build
another molecular logic design and encountered problems with parts of molecules
interfering with each other. The previous attempt copied the structure of
larger, diode-based logic circuits, said Stadler. "There we encountered
problems with quantum interferences," he said. "So we decided to try to
use those interferences for information processing rather than trying to
avoid them."

The logic scheme works because a molecule guides electrons passing
through it as quantum waves rather than as particles passing through larger
electrical circuits. "By changing the chemical side-groups, the geometry
of the waveguide can be tuned," said Stadler. Depending on how the side-groups
are rotated, they are coupled or decoupled from the cloud of electrons associated
with the benzene molecule. The side-group positions represent inputs and
two side groups provide the requisite number of inputs for the two basic
logic gates.

Changing the electronic structure of the molecule by rotating the
side groups changes the interference pattern of the different paths the
electron waves can travel through the molecule, which changes the electrical
conductance of the molecule. The conductance levels represent logic gate's
output.

There are several technical challenges to implementing even a simple
version of the molecular logic, said Stadler. The first challenge is bringing
a large number of electrodes within nanometers of each other. A nanometer
is one millionth of a millimeter. "Gap sizes of about five nanometers between
two electrodes are now possible," he said. "But for more than two electrodes
this of course becomes increasingly difficult."

A second challenge will be positioning the molecules on electrodes,
said Stadler. "In situ manipulation of the molecules when they are anchored
on electrodes is the next big hurdle," he said.

Finally, to construct a working information processing or storage
system, molecules must be interconnected. "This raises a large number of
architectural and manufacturing issues," said Stadler.

At this early stage, it is difficult to predict exactly how well
such molecular devices will work. In addition to the performance of a single
molecular system, interconnections among the molecules and a means of connection
to the larger world must be taken into consideration, said Stadler. "These
interconnections and the detailed computer architecture of the whole system
will be [the] limiting factors for performance in density rather than properties
of the molecules themselves," he said.

Practical applications for molecular electronics are more than a
decade away, said Stadler. They "should not be expected before 2015," he
said.

Even further down the road, molecular electronics could be coaxed
to interact with a chemical environment, said Stadler. "Prospects for medical
applications, where molecular devices could be linked to bio-chemical processes
would be very exciting," he said. These possibilities won't be realized
anytime soon, he added.

Stadler's research colleagues were S. Ami and Christian Joachim
at CNRS in France and Michael Forshaw at University College London. The
work appeared in the January 23, 2004 issue of Nanotechnology. The
research was funded by the European Community and the Consortium for Hamiltonian
Intramolecular Computing.